The invention relates to the field of magnetic memory, and in particular to ring-shaped memory elements having twisted states comprising 360° domain walls in the ring structure.
The use of magnetic memory that incorporates thin-film ring-shaped memory elements is known in the art. In such structures, each ring can store a bit of information depending on its magnetic state. The rings are written by applying magnetic fields (the fields are produced by passing currents through adjacent conductive lines). The data-bit in the rings is read back by detecting the rings' electrical resistance, which depends on their magnetic states. The dependence of resistance on magnetic state is called magnetoresistance. To use magnetoresistance for data readback it is most convenient to make the memory element out of a magnetic multilayer, for instance two magnetic layers separated by a non-magnetic spacer. In such a multilayer (called a spin-valve or tunnel junction), the resistance can vary by up to about 10-50% depending on the relative magnetization directions of the two magnetic layers and the structure of the multilayer.
The idea of using multilayer magnetic rings for data storage has been suggested previously, e.g., Zhu et al, J Appl. Phys. 87 6668 2000. In this case, two possible magnetic states of the rings have been identified, called “onion” and “vortex” states. The idea is to use the two vortex states to store a bit, e.g. a clockwise vortex represents a 1 and a counterclockwise vortex is a 0.
However, there is a need in the art to have a twisted magnetic state, which consists of a 360° domain wall in the ring. This state has not been reported by any other groups. It would be useful for data storage in rings, because it is quite a stable state. Also, it could allow more than one bit to be stored on a single ring. The twisted state only occurs in very small rings, which is relevant for high density storage.